The present invention relates to a display controller having therein a RAM (Random Access Memory) that stores display data and controlling a display device, further, a technique effectively applied to a display controller formed as an integrated semiconductor circuit. For example, the invention relates to a technique effectively used for a liquid crystal control semiconductor integrate circuit for driving a liquid crystal display panel.
In recent years, as a display of a portable electronic device such as a cellular phone or PDA (Personal Digital Assistant), generally, a dot-matrix-type liquid crystal panel in which a plurality of display pixels are two-dimensionally arranged in a matrix is used. In the device, a liquid crystal display controller (liquid crystal controller) formed as a semiconductor integrated circuit and controlling display of the liquid crystal panel and a liquid crystal driver for driving the liquid crystal panel under control of the controller, or a liquid crystal controller/driver having therein a liquid crystal controller and a liquid crystal driver are/is mounted.
Hitherto, in the liquid crystal controller/driver (including the liquid crystal controller), a RAM for storing display data is provided in the chip. The storage capacity of the built-in RAM is generally determined according to the size of the display screen of the liquid crystal panel to be driven and is smaller than that of a general memory. In addition, a so-called redundancy circuit for repairing a defective bit is not provided.
The reason why the storage capacity of the built-in RAM is set to the size of the screen of the liquid crystal panel is as follows. Even if the capacity of the built-in RAM is set to the size of storing display data of one screen of the liquid crystal panel in the liquid crystal controller/driver, the proportion of the RAM in the chip area is relatively large. Consequently, increase in the storage capacity directly causes increase in the chip cost. In the built-in RAM having the capacity of storing display data of one screen, deterioration in the yield due to a defect in the RAM is so significant. It is not so necessary to provide a redundancy circuit, and increase in the chip size by providing a redundancy circuit can be avoided.
The technique of setting the storage capacity of the built-in RAM to the size of storing display data of one screen of a liquid crystal panel is described in, for example, Japanese Unexamined Patent Publication No. 2000-347646.